Treatment of waste saltwater to reduce volume is becoming increasingly important, particularly for mining, oil and gas, and inland desalination systems. Mines can produce tailings water, which is typically ponded. Oil and gas operations can produce saltwater within the hydrocarbon reserve or during processing. Desalination is increasingly being used in both industries as regulations require treatment of impaired water. Desalination is also used in coastal regions to produce freshwater from seawater, with the more saline brine reject returned to the ocean. Inland brackish water can be desalted; however, there is often no convenient way to dispose of the brine reject.
Common brine reject management options include discharge to a sewer or the environment, ponding, deep well injection, or treatment to produce solid salt in concentrators and crystallizers. The first two methods are becoming more challenging to use due to tightening environmental regulations and associated costs. This is moving the emphasis to so-called Zero Liquid Discharge (ZLD) processes. In such processes, concentrators and crystallizers are used to distil water and produce solids, which can then be landfilled or put to secondary use. Due to the need to operate at high concentrations that exceed the osmotic pressure limits of reverse osmosis systems, such ZLD processes involve an evaporation-condensation cycle. In such evaporation-condensation cycles, the water in the saline saltwater waste is evaporated to a nearly pure vapor and condensed to recover nearly pure liquid water. Despite their popularity, ZLD processes are expensive, costing roughly five times as much as employing traditional deep wells. ZLD processes also experience considerable reliability challenges due to the inherent saturation point operation.
The capital costs of ZLD processes are high due to the extensive use of alloyed steels and titanium required at the operating temperatures and pressures. Energy costs are high due to the use of large volume compressors, which on average consume 20 to 60 kWh of electrical-mechanical power per cubic meter treated.
Considerable effort has been focused on multiple effect thermal desalination processes. These comprise a cascade of evaporation-condensation processes, each subsequent process in the cascade operating at a temperature below that of the previous process in the cascade. A given evaporation-condensation process in the cascade obtains its required latent heat of evaporation from the heat of condensation resulting from a preceding higher temperature evaporation-condensation process.
Humidification dehumidification (HDH) is one form of multiple effect thermal desalination, described in Canadian Patent Application 2,816,746, the entirety of which is hereby incorporated by reference herein. In brief, an air stream is humidified by warm saltwater, which drips through a humidification zone to promote heat and mass transfer from the warm saltwater to the air stream, with said air stream having a wet bulb temperature lower than the warm saltwater temperature. The humidified air stream is then cooled by a radiator surface that is colder than the wet bulb temperature of the humidified air. As air is cooled, the air's ability to hold vapor decreases and moisture condenses on the cooler tubes while transferring its heat of condensation to the cooler saltwater inside the radiator tubes, which is then directed to the humidifier of the subsequently lower temperature effect.
Operation of the system described in Canadian Patent Application 2,816,746 may be at or above saturation. However, saturated operation increases the propensity of solids and scale to build up on internal surfaces, reducing heat transfer effectiveness and clogging flow paths resulting in performance degradation and reliability challenges. In fact, the most common reoccurring maintenance required in any humidification driven saltwater concentrating system is de-scaling the system components. Therefore, there is also a need to devise a system that efficiently and periodically removes scaling in multiple effect desalination processes.
Canadian Patent Application 2,821,453, the entirety of which is hereby incorporated by reference herein, describes one such method to periodically clean internal surfaces through a series of freshwater or chemically enhanced wash cycles based on predetermined operating criteria. Additional methods, systems, and techniques for de-scaling desalination systems continue to be developed.